1. Field of the Invention
The present invention relates to a method of manufacturing a crystal resonator whereby an ultra-small crystal resonator used as a quartz crystal tuning fork, thickness-shear mode crystal resonator or gyro sensor crystal resonator using a photolithography technology and chemical etching technology.
2. Description of the Related Art
Various electronic products using parts provided with a crystal resonator have an increasing tendency to be implemented as portable devices; becoming smaller in size and being provided with higher functions in recent years. Moreover, not only a frequency signal of vibration output from a crystal resonator is used but a crystal resonator is recently used for a gyro sensor, too. For this reason, there is a limitation to a conventional manufacturing technology in satisfying the above described requirement while maintaining the characteristic of the resonator and further improvements in manufacturing processes are required.
A resonator generally used is an ultra-small crystal resonator such as a quartz crystal tuning fork and thickness-shear mode crystal resonator. One of generally used methods for manufacturing such a crystal resonator is a method using a photolithography technology and chemical etching technology disclosed, for example, in Japanese Patent Application Laid-Open No. 5-315881. Thus, the method of manufacturing this conventional small crystal resonator (quartz crystal tuning fork) will be explained using FIG. 7 and FIG. 8. FIG. 7 is a cross-sectional view of a tuning fork branching section of a crystal resonator and FIG. 8 is a cross-sectional view of the tip of the tuning fork of the crystal resonator.
First, as shown in FIG. 7(a) and FIG. 8(a), a crystal substrate 51 is cut out of a crystal raw stone at a predetermined angle and subjected to polishing to a predetermined thickness.
Then, a photoresist (not shown) is formed on both sides of the crystal substrate 51 using a spin-coating method. Then, using a predetermined photomask, it is exposed to light using a predetermined photomask, subjected to development processing and the photoresist is patterned. This patterned photoresist is used as an etching mask to carry out etching and as shown in FIG. 7(b) and FIG. 8(b), a metal film 52 made up of laminated films of chromium (Cr) and gold (Au) is formed on both sides of the crystal substrate 51 in a tuning fork shape.
Then, a photoresist is formed over the entire surface of the crystal substrate 51 including the metal film 52 using a spin-coating method, subjected to exposure to light and development processing and a photoresist 53 having an inverted shape of the surface electrode is formed on the metal film 52 as shown in FIG. 7(c) and FIG. 8(c). Furthermore, in the section corresponding to a surface space 54 between the neighboring tuning fork branching sections of the tip of the resonator, the applied photoresist 53 is left through patterning as shown in FIG. 8(c) and a side adjusting resist 59 having a shape which bridges those tuning fork branching sections is formed. As a result, the side adjusting resist 59 functions as a protective film on the inner side close to the end of the tuning fork branching section and no electrode film 56 is formed.
Then, using the photoresist 53 and metal film 52 as masks, the crystal substrate 51 is subjected to etching and a resonator shape is formed as shown in FIG. 7(d) and FIG. 8(d) (these figures only show the cross section of the branching section). At the same time, the part of the crystal substrate 51 coated with the side adjusting resist 59 is also removed through etching. Etching is applied to this part by an etching liquid wrapping around from the side, and therefore it takes more time than other parts.
Then, using the photoresist 53 as a mask, the metal film 52 is subjected to etching and a surface space 54 for formation of an electrode film is formed as shown in FIG. 7(e) and FIG. 8(e). Then, as shown in FIG. 7(f) and FIG. 8(f), an electrode film 56 is evaporated onto the entire surface including the side of the crystal substrate 51 using a vacuum deposition method, etc.
Then, as shown in FIG. 7(g) and FIG. 8(g), the photoresist 53 and the electrode film 56 formed on this photoresist 53 are removed. The photoresist 53 is removed by immersing the photoresist 53 in a heated solvent and dissolving the photoresist 53. This solvent reaches the photoresist 53 through pinholes of the electrode film 56 and dissolves the photoresist 53. At this time, the electrode film 56 formed on the crystal substrate 51 is adhered to the crystal substrate 51 and not peeled away because its adhesion is strong.
Finally, by removing the metal film 52 having the inversed pattern shape of the surface electrode through etching, a surface electrode 57 and a side electrode 58 are formed on the crystal substrate 51 (tuning fork branching section) as shown in FIG. 7(h) and FIG. 8(h). For etching of this metal film 52, the aforementioned etching liquids for gold and chromium are used. At this time, since this electrode film 56 is made of a material not etched by the etching liquids for gold and chromium, titanium and palladium, the electrode film 56 remains without being etched. As a result, each electrode is formed.
Recently, however, a detection side electrode 64 divided in the thickness direction of the resonator is often formed on the side of the resonator. An example of this is disclosed in Japanese Patent Application Laid-Open No. 10-170272 and this will be explained using FIG. 9. The crystal resonator shown in FIG. 9 comprises a drive tuning fork section 61, a detection tuning fork section 62, a tuning fork support section 63 and a detection side electrode 64. When this resonator is used as a quartz crystal tuning fork for a gyro sensor, it is necessary to lay an electrode on the drive tuning fork section 61 for exciting the resonator and the detection tuning fork section 62 for detecting a signal according to an angular velocity. This electrode is laid along four ridge lines of the crystal branching section in the longitudinal direction of the tuning fork. For this reason, in FIG. 9, bisected electrodes (detection side electrodes 64) extending in the longitudinal direction are formed on the right and left sides of the respective detection tuning fork sections 62.
This bisected detection side electrode 64 is formed by positioning and attaching a metal mask to a crystal substrate and vacuum-evaporating Cr+Au (e.g., an Au film is formed on a Cr film) metal particles onto the crystal substrate from a direction inclined by a certain angle with respect to the direction of the normal thereto.
However, it is not possible to form the bisected side electrode as shown in FIG. 9 using the conventional technology explained with reference to above described FIG. 7 and FIG. 8. That is, according to the manufacturing method shown in FIG. 7, the side electrode 58 is necessarily formed over the entire area of the crystal substrate for reasons related to the nature of the process and when the side adjusting resist 59 is bridged over the surface space 54 as shown in FIG. 8(d), no side electrode 58 is formed.
Furthermore, because the resist functions as an etching stopper, crystal etching right below the side adjusting resist 59 which functions as a bridge is prevented in the thickness direction, that is, the Z-axis direction, and therefore the etching rate is extremely slowed down compared to other locations. Therefore, the situation of progress of crystal etching becomes nonuniform between the tuning fork branching sections and it is difficult to keep the cross-sectional shape constant over the entire area of the branching section, which may cause deterioration of the characteristic of the resonator.
On the other hand, according to the method of manufacturing the crystal resonator shown in FIG. 9, the electrode is formed on the side of the resonator using a metal mask, and therefore the patterning accuracy of the electrode is determined by the machining accuracy of the metal mask body and vapor deposition jig or mechanical mask positioning accuracy of the apparatus. When an ultra-small crystal resonator requiring high precision patterning technology is manufactured, there is a limitation to the accuracy of wiring formation using the mechanical technique using a metal mask jig or apparatus. There is also a disadvantage that the process is complicated.
As is evident from the above described explanations, when an ultra-small crystal resonator is manufactured, the problem is how to make a high-precision crystal shape machining technology and a high-precision electrode patterning technology mutually compatible.
Furthermore, a method of forming an electrode bisected in the thickness direction of the substrate on the side of a thin crystal substrate of the excitation section of the crystal resonator without using any metal mask is disclosed in Japanese Patent Application Laid-Open No. 8-18371. As shown in FIG. 10, this method performs etching starting from both the upper and lower surfaces of the excitation section (crystal substrate 70) and forms etching grooves 71 with a connection section 73 having a thickness of t2 left in the center in the thickness direction of the crystal substrate 70. Then, as shown by an arrow in FIG. 10, vapor deposition is applied diagonally toward the inner surface of the grooves 71 to form a film on the electrode 72 on the inner surface of the grooves. Then, when the connection section 73 is removed, side electrodes 72 (detection electrodes) divided into upper and lower portions are formed on the sides of the crystal substrate 70.
However, with regard to the grooves 71 formed in the crystal substrate 70 through etching, the shape close to the groove bottom varies greatly depending on the temperature, concentration of the solvent for etching the crystal substrate 70, etching duration and anisotropy of etching of the crystal, and therefore the sectional shape of the grooves 71 does not become rectangular as shown in FIG. 10, but the shape actually becomes narrower near the bottom of the grooves 71 and its cross-sectional shape becomes irregular and no uniform vertical wall is formed up to the bottom of the grooves. Especially the influence of etching anisotropy of the crystal is considerable and while one side of the groove wall becomes substantially vertical, the other wall becomes sloped.
For this reason, when vapor deposition is carried out diagonally toward the inner side of the grooves 71 formed through etching, the width (size in the thickness direction of the crystal substrate 70) of the electrode 72 evaporated on the inner side is not constant. Thus, the cross section of the groove formed through etching is irregular, and therefore even if vapor deposition is applied to the inner surface, it is difficult to predict the size of the electrode to be formed or form the electrode to the same height and the same film thickness with respect to both the right and left inner surfaces.